Ultrasonic imaging has found widespread use in industrial and medical applications. Flaw detection, thickness measurement, and diagnostic imaging are just a few of the tools utilizing this technology. All information acquired by the ultrasound system passes through the transducer before being processed and presented to the operator. Therefore, the performance characteristics of the transducer can significantly influence system performance, especially when the miniaturization of ultrasonic transducer is the trend of future development. It possesses several advantages over other techniques, like x-rays or magnetic resonance imaging (MRI), including being noninvasive, relatively inexpensive, portable, and capable of producing a tomographical image—an image of a two-dimensional slice of the body. Another very important advantage is that ultrasound produces images fast enough to monitor the motion of structures within the body, such as a fetus or a beating heart. Close attention should be paid to the design and fabrication of a proper transducer for the application, taking into consideration the performance of the imaging system as a whole.
Currently, the most commonly seen ultrasonic transducers are piezoelectric ultrasonic transducers, which require ceramic manufacturing processes and have acoustic impedance similar to a solid mass that it is not suitable for operating in a gaseous or liquid ambient. Therefore, capacitive micromachined ultrasonic transducers (CMUTs) have been considered an attractive alternative to conventional piezoelectric transducers in many areas of application, since the acoustic impedance match of a CMUT to air/liquid is closer than that of piezoelectric ultrasonic transducers, due to the small mechanical impedance of the thin transducer membrane. Please refer to FIG. 29, which is a cross-sectional view of a conventional piezoelectric ultrasonic transducer. An ultrasonic transducer of FIG. 29 comprises: an layer of active element 32, which is piezo or ferroelectric material; a backing layer 30; and a wear plate 36, for protecting the transducer from the testing environment; wherein a matching layer 34 is sandwiched between the layer of active element 32 and the ware plate 36 for enhancing wave-emission efficacy.
In addition to lager operating range, better sensitivity, preferred resolution, the CMUTs also provide the following advantages over piezoelectric transducers; CMUTs can be batch produced with a standard IC process to right parameter specifications, which is difficult with lead zirconium titanate (PZT) transducers. This means that near-electronics can be integrated with the transducer. It is easier to make transducer arrays from CMUTs than from PZTs. Moreover, a CMUT can operate in a wider temperature range than a PZT device. Furthermore, the acoustic impedance match of a CMUT to air is closer than that of PZT transducers, due to the small mechanical impedance of the thin transducer membrane. That is, when both is operating in air, the operating frequency of a CMUT is in the range of 200 KHz and 5 MHZ while that of a PZT transducer is only in the range of 50 KHz and 200 KHz that the different between the operation band of the two transducers has caused troubles and restrictions in the application point of view.
A capacitive micromachined ultrasonic transducer is a device where two plate-like electrodes are biased after which an ac signal is applied on top of the dc bias to harmonically move one of the plates. The main parts of a CMUT are the cavity, the membrane and the electrode.
In 1998, a surface micromachining technique is disclosed by Jin, et al., which is adapted for fabricating a capacitive ultrasonic transducer capable of operating in air and immersed in water. The surface micromachining technique comprises the steps of: providing a high doped silicon wafer with preferred conductivity as the substrate of the transducer; depositing a first layer of nitride by low pressure chemical vapor deposition (LPCVC) at 800° C. for protecting a button electrode; depositing a layer of amorphous-silicon (a-Si) as a sacrificial layer; dry-etching the sacrificial layer to form a plurality of hexagon island; depositing a second layer of nitride to form a membrane and hexagon frames supporting the membrane; dry-etching the second layer of nitride to form via holes; removing the a-Si by feeding in KOH through the via holes at 75° C. so as to form a cavity; depositing a layer of silicon oxide to seal the via holes; plating a layer of aluminum; and patterning the layer of aluminum by wet-etching so as to form a top electrode.
In 2002, in order to deal with the membrane stress adversely affecting the performance of a capacitive transducer, a low-temperature manufacturing technique with a annealing process is developed and disclosed by Cianci, et al. for manufacturing a capacitive ultrasonic transducer, the technique comprises the steps of: provide a polymide as a sacrificial layer; etching the polymide to form a plurality of hexagon islands by means of reactive ion etch (RIE); vapor-depositing a silicon oxide to form hexagon frames, each having a height the same as that of the sacrificial layer for supporting a membrane; depositing a layer of silicon nitride to form the membrane at 380° C. by plasma enhanced chemical vapor deposition (PECVD); and annealing at 510° C. for 10 hours for eliminating the compressive stress of the membrane while conserving only a slight tensile stress thereof. Moreover, the top electrode of the transducer is formed by lithographic patterning while the button electrode is being plated at the back of the silicon wafer.
However, the two aforesaid manufacturing techniques all have problems of high processing temperature, high residue stress, uncontrollable features of manufacturing, and high cost, and thus a certain corresponding procedures should be adopted in the manufacturing processes for resolving the foregoing problems, such as adopting an annealing procedure to reduce the residue stress so as to prevent the membrane to be damaged by deformation. In addition, most prior-art transducers have their cavity of excitation formed by etching a silicon-based material, such that an obvious Lamb Wave effect can occur, and furthermore, the ultrasonic transducer can be in an unstable state since the cavity and the membrane thereof are made of different materials which have different thermal expansion coefficient.
Therefore, it is in great demand to have a polymer-based capacitive ultrasonic transducer capable of overcoming the shortcomings of any prior-art transducers.